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CNG Buses

Compressed Natural Gas

Summary

Buses are one of the most cost-effective forms of public transport and play an integral role in the public transport systems in most world cities. Diesel engines have been the traditional power source for public buses due to their durability, robustness, reliability, high fuel efficiency and high torque. Posada (2009) describes how the 1990’s saw the provision of several motivations for an increased interest in the use of compressed natural gas (CNG) buses around the world. Posada (2009) notes that the need for significant emission reductions from urban buses, whose stop-and-go traffic patterns in heavily congested areas contribute to serious urban air pollution and health problems, and the desire to use alternative fuels to offset growing oil imports stimulated a growing CNG bus market in both the European Union and United States. The European Union through the EURO programme and the United States have been introducing successively tighter emissions standards These stimuli had the effects of changing some urban bus fleets from fleets of pure diesel powered buses to combined fleets of diesel buses with emission reduction technologies and compressed natural gas (CNG) buses. The predictability of urban bus travel patterns and daily travelled distances along with centralised depots and well developed local gas networks provide an added attractiveness for developing urban CNG bus networks in many casaes. According to the International Association of Public Transport (2005) in 2005 diesel buses represented 90.6% (67,200) of Europe’s urban fleet with CNG buses accounting for 5.8% (4,060). In the United States the US Department of Energy (2009) describe how CNG buses totalled 4,550 or 7% of the urban bus fleet in 2007.

Implementation

Natural gas is a fossil fuel that is traditionally commercially produced from natural gas fields or oil fields. Natural gas is composed primarily of methane (87%-96% [Posada, 2009] ) and can be used as combustion energy source, acting as a fossil fuel substitute for gasoline or diesel. While the combustion of natural gas will still produce greenhouse house gases, it is often viewed as a more environmentally clean alternative to other fossil fuels. Natural gas vehicles (NGVs) will have much lower non-methane hydrocarbon emissions than gasoline vehicles although methane emissions will naturally be higher. When compared against conventional diesel engine vehicles using high sulphur fuels NGVs have traditionally had somewhat lower NOx and significantly lower PM emissions (see Clean Air Initiative).

Compressed natural gas (CNG) is generated when natural gas is compressed at high pressure (200 – 220 bar) and it is then stored and distributed in hard containers, usually in cylindrical or spherical shapes. According to Posada (2009) this process reduces natural gas volume by 99% compared to standard atmospheric conditions, thus allowing for significantly greater driving ranges between fuelling points. CNG buses will have vehicle mounted CNG cylinders installed for (rapid) refuelling. CNG buses require a greater amount of space for fuel storage relative to traditional diesel powered buses. Since CNG is a compressed gas rather than a liquid like diesel CNG utilises more space for each diesel gallon equivalent. For urban buses CNG cylinders are typically installed on bus roofs due to the availability of large space (refer to bus roof picture). A significant challenge for CNG bus deployment is the lack of CNG refuelling infrastructure. A major issue with CNG operationalisation is the need to compress the natural gas from the urban distribution system to pressures used in vehicle-mounted cylinders for rapid refuelling. As a result a purpose built compression plant with storage capability is thus required at the operating urban bus depot. In order to retain competitiveness with diesel buses CNG operators need to ensure that the refuelling process is as efficient and rapid as possible. The US Federal Transit Administration (2009) describes fuelling time for diesel buses as ranging from 2.5 – 10 minutes. CNG refuelling technology has developed to a stage where it now exists on a par with diesel refuelling. In 2009 Ballast Nedam built a CNG bus refuelling station in Njimegen in the Netherlands with capacity for refuelling CNG buses in 7 minutes. Smyth (2008) refers to CNG bus refuelling times of 8-10 minutes at Turin CNG bus terminal and filling station.

The core driving dynamics of a CNG bus consist of a diesel engine modified to a spark-ignition engine optimised for the use of compressed natural gas CNG bus engines generally take one of two forms:

• Stoichiometric engine : Stoichiometric combustion is a combustion process in which fuel and oxygen are completely consumed with no unburned fuel or oxygen in the exhaust. Stoichiometric combustion allows for the use of a three-way catalytic converter as in common gasoline cars. A three-way catalytic converter allows for the control of NOx, HC and CO.
• Lean burn engine: Lean burn combustion is achieved with excess air in the combustion chamber with the resulting exhaust containing significant oxygen. The lean burn engine does not allow for the use of a three-way catalytic converter although oxidation catalysts can be utilised. This results in lean burn engines having higher NOx emissions relative to the stoichiometric engine.
• Since CNG fuel systems are closed, there are no evaporative emissions and refuelling emissions are negligible. Cold-start emissions from CNG engines are also low since cold-start enrichment is not required; thereby reducing both VOC and CO emissions. PM emissions from both stoichiometric and lean burn CNG engines are very low due to the almost homogenous combustion of the air-gas mixture, and the absence of large hydrocarbon chains in the fuel.

According to both Posada (2009) and the OECD (1999) CNG bus engine types were originally dominated by the lean burn engine due to its higher fuel efficiency and ability to provide power and torque levels equivalent to diesel engines of similar displacement. The OECD (1999) note how a fuel burn engine is some 10-20% more fuel efficient than the stoichiometric engine. However, Posada (2009) highlights that more stringent NOx emission limits from US 2010 and Euro V forced some engine manufacturers to choose stoichiometric combustion technology for CNG engines because of the ability to use three-way catalyst technology.

While lean burn engines are more fuel efficient than stoichiometric engines, CNG buses in general are less fuel efficient than traditional diesel buses. Austin et al. (2000) and NYBD (2000) cite how CNG buses are between 17% and 41% less fuel efficient than conventional diesel buses. In a study of New York CNG transit buses the National Renewable Energy Laboratory (2006) found the average fuel economy for CNG buses to be 25% lower compared to studied diesel buses. The Clean Air Initiative website notes that CNG buses also have a lower driving range than diesel buses with CNG buses described as having a driving range of about 300 miles depending upon the capacity of the gas cylinders) compared to a little more than 400 miles for diesel buses.

The main barriers to CNG bus deployment are their high capital costs and lack of refuelling infrastructure. The above factors combine to ensure that the capital costs associated with CNG bus and equipment purchase (gas cylinders, piping, valves etc) are more expensive relative to diesel buses. The International Association of Public Transport (2009) detailed that in 2006 a CNG powered 12-metre transit bus cost approximately 15-20% more than a diesel bus in Europe. Posada (2009) provides a price comparison of CNG and diesel buses between 2005 and 2007. Referring to the American Public Transportation Association Transit Vehicle Database Posada notes that diesel bus purchase prices ranged from $257,000 to $416,000 with a weighted average of $329,500 over the 2005-2007 period. Over the same period CNG bus prices ranged from $329,000 to $410,000 with a weighted average cost of $376,000. On average CNG buses were approximately $46,000 more expensive than diesel buses. In addition initial infrastructural investment will be significant. To achieve economies of scale CNG vehicle purchase and fuelling and service infrastructure should ideally be dealt with together in a coordinated approach. An article by Petersen of the Wuppertal Institute for the Clean Air Initiative indicates that a shift from diesel to CNG in urban bus companies demands comprehensive planning. He notes that an investment in a minimum of between 20 to 50 CNG buses seems to be necessary to justify investment in a CNG fleet.

Even though the cost of CNG vehicles is higher than diesel vehicles, due to both market size differences and a more complicated fuel storage and delivery system, the CNG solution for mass urban transportation is a technically feasible and economically viable way to comply with the most stringent emission regulations. Higher CNG capital costs can be offset or reduced through increased market penetration. In addition high CNG costs may be offset by natural gas price differential to diesel and fossil fuel tax rates. a significant cost difference between CNG and diesel would reduce operating costs and offset initial capital expenditure.

Examples of cities where CNG buses make up part of urban bus fleets in Beijing, Delhi, Los Angeles, Mexico City, Miami, New York, Pakistan, Paris, Prague, Turin, Vancouver and Zagreb. It is important to note that the introduction of new more stringent emissions standards for new engines in recent times in Europe and the US provided an R&D stimulus for the development of new diesel engines for heavy duty vehicles. Posada (2009) notes that this has had the impact of making new advanced diesel engines more competitive with CNG engines from an emissions standpoint. This development may explain why CNG buses have failed to achieve greater penetration of urban fleets than the levels described earlier in this section. However, Petersen of the Wuppertal Institute describes that future CNG engine technology development has the potential to improve CNG bus fuel efficiency and driving range thus reducing diesel bus advantage and improving CNG attractiveness.

Impact

Where CNG buses are introduced to urban bus fleets to replace existing diesel models there exists the potential for this investment to bring about a reduction in emission levels relative to conventional diesel buses. The Costs and Benefits entry for CNG provides details on the potential emissions impact.

Costs & Benefits

The costs associated with CNG bus deployment fall into two basic categories – capital costs for CNG bus purchase and infrastructure development and CNG bus fleet maintenance costs. Data obtained by Posada (2009) from the American Public Transportation Association Transit Vehicle Database reveals that between 2005 and 2007 CNG bus prices ranged from $329,000 to $410,000 with a weighted average cost of $376,000 in the US.

Principal benefits associated with CNG buses are reduced emissions relative to diesel buses and improvements in air quality. As stated in the implementation new diesel engines are more competitive with CNG engines from an emissions point of view. The professed emissions benefits of CNG engines are based on comparison with traditional diesel engines.

Evidence & Reference

Modelling this Measure

In this case we consider the modelling of this measure under three headings related to the most relevant modelling parameters

1. Demand Factor
The introduction of CNG buses into the urban bus fleet will have one of two influences on fuels used by the urban bus fleet. Where CNG buses are introduced in addition to the existing bus fleet natural gas consumption levels within national activity data will increase. Where CNG buses are introduced to replace part of the existing diesel fleet diesel use levels will decline with natural gas consumption increasing.

2. Control Factor
The impact of CNG bus deployment on control data will depend on the number of CNG buses introduced along with the impact that such a policy measure has on the number of diesel buses in the urban fleet in terms of replacement or renewed future diesel investment.

3. Switch Factor
The perceived greenhouse gas emissions benefits of CNG buses could attract private transport users to use the bus through a so called “green appeal”, thereby resulting in a modal shift.